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LT3991-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LT3991-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LT3991/LT3991-3.3/LT3991-5 11 3991fa applicaTions inForMaTion Operating Frequency Tradeoffs Selection of the operating frequency is a tradeoff between efficiency,componentsize,minimumdropoutvoltage,and maximum input voltage. The advantage of high frequency operationisthatsmallerinductorandcapacitorvaluesmay be used. The disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. The highest acceptable switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX) = VOUT + VD tON(MIN)(VIN − VSW + VD) where VIN is the typical input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), and VSW is the internal switch drop (~0.5V at max load). This equation shows that slower switching frequency is necessary to safely accommodate high VIN/VOUT ratio. Also, as shown in the Input Voltage Range section, lower frequency allows a lower dropout voltage. The input voltage range depends ontheswitchingfrequencybecausetheLT3991switchhas finite minimum on and off times. The minimum switch on and off times are strong functions of temperature. Use the typical minimum on and off curves to design for an application’s maximum temperature, while adding about 30%forpart-to-partvariation.Theminimumandmaximum duty cycles that can be achieved taking minimum on and off times into account are: DCMIN = fSWtON(MIN) DCMAX = 1− fSWtOFF(MIN) where fSW is the switching frequency, the tON(MIN) is the minimumswitchon-time,andthetOFF(MIN)istheminimum switch off-time. These equations show that duty cycle range increases when switching frequency is decreased. See the Electrical Characteristics section for tON(MIN) and tOFF(MIN) values. A good choice of switching frequency should allow ad- equate input voltage range (see Input Voltage Range sec- tion) and keep the inductor and capacitor values small. Input Voltage Range The minimum input voltage is determined by either the LT3991’s minimum operating voltage of 4.3V or by its maximumdutycycle(seeequationinOperatingFrequency Tradeoffs section). The minimum input voltage due to duty cycle is: VIN(MIN) = VOUT + VD 1− fSWtOFF(MIN) − VD + VSW where VIN(MIN) is the minimum input voltage, VOUT is the output voltage, VD is the catch diode drop (~0.5V), VSW is the internal switch drop (~0.5V at max load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time. Note that higher switch- ing frequency will increase the minimum input voltage. If a lower dropout voltage is desired, a lower switching frequency should be used. The maximum input voltage for LT3991 applications depends on switching frequency, the Absolute Maximum Ratings of the VIN and BOOST pins, and the operating mode. For a given application where the switching fre- quency and the output voltage are already selected, the maximum input voltage (VIN(OP-MAX)) that guarantees optimum output voltage ripple for that application can be found by applying the following equation: VIN(OP-MAX) = VOUT + VD fSW • tON(MIN) – VD + VSW where tON(MIN) is the minimum switch on-time. Note that a higher switching frequency will decrease the maximum operating input voltage. Conversely, a lower switching frequency will be necessary to achieve normal operation at higher input voltages. The circuit will tolerate inputs above the maximum op- erating input voltage and up to the Absolute Maximum Ratings of the VIN and BOOST pins, regardless of chosen switching frequency. However, during such transients where VIN is higher than VIN(OP-MAX), the LT3991 will enter pulse-skippingoperationwheresomeswitchingpulsesare skipped to maintain output regulation. The output voltage ripple and inductor current ripple will be higher than in typical operation. Do not overload when VIN is greater than VIN(OP-MAX). |
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